High sensitivity of tropical precipitation to local sea-surface temperature.

Precipitation and atmospheric circulation are the coupled processes through which tropical ocean surface temperatures drive global weather and climate1-5. Local ocean surface warming tends to increase precipitation, but this local control is hard to disentangle from remote effects of conditions elsewhere. Such remote effects occur, for example, from El Nino Southern Oscillation (ENSO) events in the equatorial Pacific, which alter precipitation across the tropics. Atmospheric circulations associated with tropical precipitation are predominantly deep, extending up to the tropopause. Shallow atmospheric circulations6-8 impacting the lower troposphere also occur, but the importance of their interaction with precipitation is unclear. Uncertainty in precipitation observations9,10 and limited observations of shallow circulations11 further obstruct understanding of the ocean's influence on weather and climate. Despite decades of research, persistent biases remain in many numerical model simulations12-18, including excessively wide tropical rainbands14,18, the 'double-intertropical convergence zone (ITCZ) problem'12,16,17 and too-weak responses to ENSO15. These demonstrate stubborn gaps in our understanding, reducing confidence in forecasts and projections. Here we show that the real world has a high sensitivity of seasonal tropical precipitation to local sea-surface temperature. Our best observational estimate is 80% precipitation change per g/kg change in the saturation specific humidity (itself a function of the ocean surface temperature). This observed sensitivity is higher than in 43 of the 47 climate models studied, and is associated with strong shallow circulations. Models with more realistic sensitivity have smaller biases across a wide range of metrics. Our results apply to both temporal and spatial variation, over regions where climatological precipitation is around 1 millimetre per day or greater. Novel analysis of multiple independent observations, physical constraints and model data, underpin these findings. The spread in model behaviour is further linked to differences in shallow convection, providing a focus for accelerated research, to improve seasonal forecasts through multidecadal climate projections.